DOMESTIC HEATING
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Understanding heat pump cylinders
Heat pump cylinders represent a signifi cant evolution from traditional boiler-based setups in modern home heating systems. While they may appear outwardly similar to standard hot water storage tanks, their internal mechanics are designed to maximise the effi ciency of heat pump technology. Kevin Lowe, technical manager, Hot Water Association (HWA), explains what heat pump cylinders are, explores the concept of integrated heat pump cylinders, and underscores the critical importance of correctly sizing these systems
Left: Kevin Lowe, technical manager, Hot Water Association (HWA)
Integrated heat pump cylinders A
t fi rst glance, a heat pump cylinder resembles the conventional storage tanks commonly found in homes today: a sleek
white casing with plastic end caps and control interfaces. However, the crucial diff erences are inside. Unlike traditional boilers that operate at higher fl ow temperatures, heat pumps work at lower temperatures, requiring a more effi cient heat exchanger to extract maximum heat from the source. Additionally, due to higher fl ow rates typical in heat pump systems, the coils inside these cylinders are often larger in diameter. This design minimises pressure resistance, ensuring optimised fl ow rates and reducing system noise and turbulence, factors that could cause annoyance in the home.
When heating water, heat pumps achieve lower
temperatures compared to boilers. This means water at the tap may be slightly cooler, typically around 50-52°C due to the nature of heat exchange, which is rarely 100% effi cient. However, this is still a suitable temperature for washing up, showering, etc. To mitigate risks associated with Legionella bacteria, which thrives between 20°C and 45°C, heat pump cylinders carry out regular pasteurisation cycles. These cycles usually use the electric immersion heater to boost the water temperature to over 60°C, maintaining water hygiene without constantly using direct electricity, which could off set any energy savings from the heat pump.
28 July 2025
An integrated heat pump cylinder combines a hot water tank and a dedicated heat pump unit into one appliance. These systems are increasingly popular, particularly in well-insulated new-build homes or hybrid heating systems where the main space heating is provided by a traditional boiler. In most cases, the heat pump unit is located on top of the cylinder, with the refrigerant pipework
wrapped around the outside of the tank, within the insulation layer, to heat the stored water. This design avoids the need for an immersed coil, which can pose risks if refrigerant leaks into the water. The heat will take the path of least resistance, passing through the metal of both the coil and the cylinder wall, rather than through the insulation. Integrated cylinders are primarily designed solely
for hot water generation, not space heating. They off er high effi ciency and convenience, but they also require adequate ducting for airfl ow, which can sometimes be a challenge in retrofi t installations. Properly designed, however, they are particularly well suited for commercial premises and modern homes where hot water demand often exceeds space heating needs.
The importance of correct sizing
One of the key challenges with heat pump systems is that they are far less forgiving than boilers when
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